Telecommunications circuits are typically interconnected using aerial cable strung between telephone poles. This type of installation results in telecommunications circuits which are susceptible to lightning strikes. For this reason, telecommunications equipment that might be connected to these lines must employ some method of resisting damage that may be caused by lightning surges entering the equipment by way of the telecommunications line. Various regulatory bodies, as well as telephone provider equipment standards, require this immunity be demonstrated in the form of a formal test report for this type of equipment.
Traditionally, surge immunity in telecommunications equipment is achieved by two primary methods: insulation (spacing) and use of surge-suppressor devices (gas tube, solid state, etc.).
The insulation method depends on an insulating barrier between the telecommunications circuitry in the equipment and other circuitry which may not be capable of withstanding lightning surge energy such as low-voltage logic, etc. The voltages which must be insulated are on the order of many thousands of volts, and therefore the insulation must be very thick in order to withstand these voltages. For a standard printed wiring board, 15 mm of spacing between conductors may be required.
As the industry moves towards miniaturization and requires more compact designs than are practical with the insulation method, the use of surge suppression devices has become commonplace. These devices are intended to intentionally break down at a particular low voltages and shunt this lightning energy safely to ground, thus protecting the more sensitive electronics in the device from high voltages. Devices like this are employed on each telecommunication-line conductor (tip and ring) where they enter into the equipment, connecting between tip and ring and ground.
In principle, any xe2x80x9cspark gapxe2x80x9d, consisting of an energized conductor placed sufficiently close to a ground conductor, will function as a surge suppressor. At some voltage, depending on the distance between conductors, the insulation offered by air that separates these conductors is broken down, resulting in an arc. Current flows through this arc and the voltage is therefore held low.
Pressure to produce higher-density telecommunications equipment continues to push towards more compact designs. The limits of space and the size of surge suppression devices dictate the maximum density of telecommunications devices. The result is typically a balance of lightning immunity and miniaturization.
Surge suppression devices, while more compact than insulation, still have a limit to how compact they can be, which is defined in large part by the distance which must be present between their pins in order to provide insulation at voltages below their activating voltage. Also, there is a cost associated in procuring these components and placing them on a printed wiring board. In order to provide reliable surge suppression, each telecommunications line must be equipped with at least two of these suppressors, one for tip and one for ring. For example, for a 256-channel DSL Multiplexer, at least 512 of these devices must be employed in order to provide reliable lightning suppression. This is expensive in terms of space and cost.
Furthermore, the value in making one product which may be supplied to a global marketplace is high, and therefore a push is made to conform to requirements for all countries. Certain European and Australian requirements specify design constraints that appear to be at odds with this surge suppressor technique. One such requirement is that the device be capable of withstanding certain types of lightning strikes without any kind of breakdown. Another requirement is that telecommunication circuits do not employ suppressors connected to ground in order to achieve immunity.
These requirements tend to force equipment makers into either excluding these countries from their marketplace, into making different equipment for each marketplace, or into the use of the insulation method which is undesirable due to excessive space requirements.
Accordingly, a need has arisen for a method and apparatus for lightning suppression in a telecommunications printed circuit board. The present invention provides a method and apparatus for lightning suppression in a telecommunications circuit board that addresses shortcomings of prior systems and methods.
According to one embodiment of the invention, a printed circuit board includes at least one telecommunications circuit conductor operable to connect to a telecommunications line and a ground conductor for electrically grounding the printed circuit board. The printed circuit board also includes at least one other conductor to be protected from lightning and a floating element electrically floating and disposed proximate the telecommunications circuit conductor and also disposed proximate the ground conductor. The floating element is operable to induce a lightning charge to flow to the ground conductor through the floating element and not flow to the at least one other conductor.
According to another embodiment of the invention, a method for lightning protection for a printed circuit board includes providing a printed circuit board having a plurality of conductors for connecting to a plurality of telecommunications lines, surrounding a portion of the conductors with a floating element to induce any lightning received by the conductors to flow to the floating element, and disposing a ground conductor with respect to a portion of the floating element such that lightning will be induced to flow to ground from the floating element.
Embodiments of the invention provide numerous technical advantages. For example, some embodiments of the invention allow lightning protection is a printed circuit board having a plurality of conductors susceptible to lightning. Such lightning protection may be achieved in a low-cost manner by providing a floating element at an appropriate location on the printed circuit board that satisfies pertinent regulations while achieving lightning protection. Such lightning protection results in more reliable printed circuit boards that comply with pertinent regulatory body standards. Additionally, printed circuit boards according to the teachings of the invention reduce risk to users who are normally exposed to low voltage signals, but do not expect to come into contact with high voltage signals.
Other technical advantages are readily apparent to one skilled in the art from the following figures, descriptions, and claims.